1. Field of the Invention
The present invention relates to electromagnetic transducers of the moving coil type.
2. Background Information
Such transducers are used in telephone subsets and their important features should be:
good acoustic sensitivity PA0 very low sensitivity to external magnetic fields PA0 high insulation from electrical fields between the front cover and the terminals, and PA0 low price. PA0 improves magnetic shielding PA0 represents the acoustic inlet impedance PA0 protects the sensitive diaphragm mechanically PA0 holds the magnet system assembled, and PA0 increases the electric flash-over or insulation voltage from the front cover to the terminals.
The main problem with known transducers and microphones of the moving coil type is that they are too sensitive to external magnetic ac fields.
A transducer of this type and which is described in U.S. Pat. No. 4,418,247 (K. B. Hansen 8) has an acoustic SFERT weighed sensitivity of S=-53 dB relatively to 1V/PA. This transducer has an immunity to external ac fields which is better than the average moving coil transducers on the market.
The sensitivity to external magnetic fields on the order of 10.sup.-7 T and in the frequency range 50 Hz to 4 kHz is as follows: EQU P&lt;S-35 dB=-53-35 dB=-88dB.
The main object of the present invention is to improve this attenuation picture.
A further problem with known telephone handsets is that the microphones built into the set are too sensitive towards static electricity. Due to the lack of space in the microphone end of the handset and due to the desired acoustical sensitivity, the distance from the outer side of the microphone end of the handset to the front of the handset is usually very small. In some countries telephone subsets shall withstand a test voltage of 15 kV between the outside of the microphone end of the handset and the terminals of the subset. This means that the transducer taken alone should show an insulation voltage of some 12 kV from its front to its terminals.